This invention relates to an inverter, a power supply system and a method of reducing leakage current in the power supply system. More particularly, the invention relates to a reduction in the leakage current of a power supply system that converts, to AC power, the output of a DC power supply such as a solar battery having an earth capacitance (capacitance to ground).
Environmental problems that are becoming increasingly serious include global warming, which is caused by the discharge of gases such as carbon dioxide that accompanies the use of fossil fuels, and radioactive contamination caused by accidents at atomic power plants and the radioactive waste. Hence there is growing interest in the global environment and energy resources. Under these circumstances, research and development concerning cleaner new energy sources is being promoted in view of this environmental awareness and solar power generation systems that employ solar batteries for converting the solar energy of the sun directly to electrical energy are becoming increasingly popular. One of these systems that is becoming particularly widespread in the market is a solar power generation system in which DC power generated by a solar battery is converted into AC power by an inverter for cooperation with a commercial power system, with the AC power obtained being output to the commercial power system.
FIG. 6 illustrates an example of the structure of a solar power generation system connected to an ordinary commercial power system. A solar battery array 1 is constructed by combining series-parallel-connected solar battery modules that provide a desired voltage and current. DC power output from the solar battery array 1 is input to an inverter 2, which converts the DC power to AC power. The AC power enters a commercial AC power system 3 via an earth leakage breaker 30.
To achieve higher efficiency, smaller size, lighter weight and lower cost in modern solar power generation systems, there is increasing use of non-insulated type, so-called transformerless inverters that do not possess an isolating transformer. FIG. 6 illustrates an example of such an arrangement, which is obtained by combining a chopper circuit and a bridge circuit. More specifically, the system includes main circuits such as an input smoothing capacitor 6, a chopper-type converter circuit 7, an intermediate smoothing capacitor 8, a bridge-type inverter circuit 9, an interconnected reactor (linking reactor) 10 and an interconnected relay (linking relay) 11, various detection circuits such as an input-voltage detector 12 for detecting the input voltage of the inverter 2 and an intermediate-voltage detector 13 for detecting the voltage of the intermediate smoothing capacitor, and a controller 14 for controlling the main circuits on the basis of signals from the detector.
The input voltage is adjusted by an input-voltage control 15 to obtain a prescribed target input voltage Vixe2x80x2 by well-known MPPT control, the adjusted voltage is input to a converter circuit driver 16, whereby there is produced a PWM signal for driving a switching element 7Q in the converter circuit 7. The switching element 7Q is driven by the PWM signal, whereby control is performed in such a manner that the input voltage is rendered constant. The intermediate voltage also is controlled so as to obtain a prescribed voltage higher than a peak voltage of the AC voltage capable of being transmitted to the commercial power system.
The wiring on the customer side is provided between an earth leakage breaker 30 and the inverter 2. The leakage breaker 30 senses a ground-fault accident at the customer by detecting a ground-fault current and interrupts the connection between the customer and the outside in such a manner that the effects of the accident will not extend to the commercial AC power system 3, which is external to the customer.
The solar battery array 1 has an earth capacitance 4 because of its large area. Though modules integrated with building materials and modules of reduced thickness have been developed as the solar battery modules that construct the solar battery array 1, there are also cases where metal plates (material exhibiting electrical conductivity) are used as reinforcing materials and cases where metal plates are tiled over underlying materials. Since the solar batteries and the metal plates oppose each other over a wide area with a short distance between them, there are instances where the earth capacitance 4 is large. Further, if moisture due to rain or the like attaches itself to the surface of the solar battery modules, the earth capacitance 4 may be produced via the moisture.
If the potential to ground of the solar battery array 1 should happen to fluctuate for some reason in a case where the earth capacitance 4 of the solar battery array 1 is large and the inverter 2 is of the transformerless type, the ground-fault current will produce a leakage current in the path that consists of the earth capacitance 4, a resistance component 5, ground, the commercial AC power system 3, the leakage breaker 30 and the inverter 2. The value of this leakage current increases in accordance with the earth capacitance 4.
If the value of this leakage current exceeds a threshold value detected in the leakage breaker 30, the leakage breaker 30 will operate and shut down power at the customer regardless of the fact that ground fault has not occurred at the customer.
This problem is not limited to a solar power generation system. There is the possibility that a similar problem will occur if the earth capacitance (earth stray capacitance) becomes too large in a power generation system that uses a power supply for generating DC power, such as a battery or fuel cell.
Accordingly, an object of the present invention is to provide a non-insulated type inverter in which leakage current is reduced.
Another object of the present invention is to reduce leakage current in a power supply system that converts the output of a DC power supply having an earth capacitance to AC power.
Further object of the present invention is to provide a method of reducing leakage current in a power supply system having a non-insulated type inverter.
According to a first aspect of the present invention, the above objects are attained by providing a non-insulated type inverter for converting DC power to AC power and outputting the AC power to a single-phase three-wire AC power system having a grounded neutral point, comprising: a converter circuit for boosting voltage of DC power that has entered from two terminals, an inverter circuit for converting DC voltage, which has been boosted by the converter circuit, to alternating current, a detector for detecting AC components of voltages to ground produced at respective ones of the two terminals, and a controller for reducing the AC components of the DC voltage, which is input to the inverter circuit, based upon outputs of the detector.
According to a second aspect of the present invention, the above and other objects are attained by providing a non-insulated type inverter for converting DC power to AC power and outputting the AC power to a single-phase three-wire AC power system having a grounded neutral point, comprising: a converter circuit for boosting voltage of entered DC power, an inverter circuit for converting DC voltage, which has been boosted by the converter circuit, to alternating current, an intermediate smoothing capacitor provided between the converter circuit and the inverter circuit, a first detector for detecting an AC component of DC voltage that is input to the converter circuit, a second detector for detecting an AC component of voltage at both ends of the intermediate smoothing capacitor, and a controller for controlling the DC voltage input to the converter circuit in such a manner that the two AC components detected by the first and second detectors will be synchronized.
Thus, according to the first aspect of the present invention, there is provided a non-insulated type inverter having a converter circuit for boosting the voltage of DC power that has entered from two terminals, and an inverter circuit for converting the DC voltage, which has been boosted by the converter circuit, to alternating current, the DC power being converted to AC power which is then output to a single-phase three-wire AC power system having a grounded neutral point, wherein AC components of voltage to ground produced in each of the two terminals are detected and AC components of the DC voltage that enters the inverter circuit are reduced based upon results of detection.
As a result of this arrangement, AC components (ripple) contained in the DC power that is input to the converter is cancelled out in the vicinity of the input terminals and leakage current can be reduced.
According to another aspect of the present invention, there is provided a non-insulated type inverter having a converter circuit for boosting the voltage of DC power input thereto, an inverter circuit for converting the DC voltage, which has been boosted by the converter circuit, to alternating current, and an intermediate smoothing capacitor provided between the converter circuit and the inverter circuit, the DC power being converted to AC power which is then output to a single-phase three-wire AC power system having a grounded neutral point, wherein an AC component of DC voltage input to the converter circuit and an AC component of voltage at both ends of the intermediate smoothing capacitor are detected and the DC voltage that enters the converter circuit is controlled in such a manner that the two AC components will be synchronized.
In accordance with this arrangement, a ripple component of the input voltage and a ripple component of the intermediate voltage become substantially equal, as a result of which it is possible to null leakage current, which flows externally, caused by ripple in the intermediate smoothing capacitor.
The above and other objects are attained by a power supply system that includes a DC power supply having an earth capacitance, and the above-described inverter.
Further, according to a third aspect of the present invention, the above and other objects are attained by providing a method of reducing leakage current of a power supply system having a non-insulated type inverter which includes a converter circuit for boosting voltage of DC power that has entered from two terminals, and an inverter circuit for converting the DC voltage, which has been boosted by the converter circuit, to alternating current, the inverter converting DC powder to AC power and outputting the AC power to a single-phase, three-wire AC power system having a neutral point that is connected to ground, the method comprising the steps of: detecting AC components of voltages to ground produced at respective ones of the two terminals, and reducing the AC components of the DC voltage, which is input to the inverter circuit, based upon results of detection.
Further, according to a fourth aspect of the present invention, the above and other objects are attained by providing a method of reducing leakage current of a power supply system having a non-insulated type inverter which includes a converter circuit for boosting voltage of entered DC power, an inverter circuit for converting DC voltage, which has been boosted by the converter circuit, to alternating current, and an intermediate smoothing capacitor provided between the converter circuit and the inverter circuit, the inverter converting DC powder to AC power and outputting the AC power to a single-phase, three-wire AC power system having a neutral point that is connected to ground, the method comprising the steps of: detecting an AC component of DC voltage that is input to the converter circuit and an AC component of voltage at both ends of the intermediate smoothing capacitor, and controlling the DC voltage input to the converter circuit in such a manner that the two AC components will be synchronized.
Other features and advantages of the present invention will be apparent from the following description taken in conjunction with the accompanying drawings, in which like reference characters designate the same or similar parts throughout the figures thereof.